1. Field of the Invention
This invention relates to an adjustable clearance control for rotor blade tips in a gas turbine engine, particularly a control that employs a radially adjustable rub strip for said rotor blade tips.
2. The Prior Art
In conventional gas turbine engines, tip clearance of rotor blades with the housing walls changes with engine speed as well as with rotor blade and housing temperatures. Yet close blade tip clearance with the housing walls is desirable to minimize engine thrust and efficiency losses.
To deal with the problem, prior art designers have provided an abradable rub strip mounted to the engine walls in close clearance with the rotor blade tips that follow the path defined by such tips in rotation.
Taking for example, compressor blades in a gas turbine engine and referring to drawings of the prior art in FIGS. 2 and 3 hereof, gas turbine engine 10 has engine housing walls 12 and 14, compressor rotor 16, compressor blades 18, blade tips 20 and a fixed rub strip 22 as shown in FIGS. 2 and 3.
As indicated in FIG. 3, on a power surge, e.g. an "accel", the compressor blades 18 deflect and/or lengthen, biting into the fixed rub strip 22 and abrading same, as shown in FIG. 3. When the engine operates at reduced power e.g. on a decel, the compressor blades 18 retract e.g. to the dotted line indicated at 24 in FIG. 3, with a pronounced increase between clearance or gap between blade tip 20 and rub strip 22 with resulting engine efficiency and thrust losses.
Attempts have been made in the prior art to provide a rub strip concentrically mounted with rotor blades, which rub strip radially expands or contracts in response to rotor blades which expand or contract. See for examples, U.S. Pat. No. 4,683,716 to Wright (1987). Wright employs an expandable metal chamber mechanism which can be pressurized and evacuated to expand or contract the outer case wall and cause a change in the rotor tip clearance. However, this mechanism has only two positions, fully expanded and fully contracted, thus limiting engine operating conditions.
Another prior art reference is U.S. Pat. No. 4,657,479 to Brown et al. (1987). Brown employs a segmented shroud which expands and contracts circumferentially by a spring, pin and cam mechanism.
However, neither of the above references employs a variable diameter rub strip which can automatically and appropriately respond for all engine flow conditions, e.g. of engine surge or stall.
Accordingly, there is a need and market for an expandable and contractible blade tip rub strip for gas turbine engine that overcomes the above prior art shortcomings.
There has now been discovered a rub strip for rotor tip blades that is radially expandable and contractible, in keeping with the rotor blade length at various engine operating conditions, which rub strip can be programmed to respond or can automatically respond to changes in blade length at various engine operating conditions, to maintain a suitable clearance or gap with the tips of such blades.